Both annular dies and methods for extruding tubular preforms of the type described above are known from various prior-art publications. For example, a device and a method are known from DE 199 29 381 D2, the latter document being concerned in particular with the setting of an annular die for producing tubular performs, which are made to expand in a blow mold to form plastic containers. A setting is in any case always required whenever hollow bodies with different cross sections, different weights, etc. are produced. In other words, the setting is generally a one-off process that is performed product-specifically.
To be differentiated from this is the wall thickness control during the extrusion. This wall thickness control over the length and circumference of the tubular or sheet-like extrudate is likewise known in principle in the prior art. Here a distinction is drawn between axial wall thickness control (known as WDS) and radial wall thickness control (known as PWDS). In the case of axial wall thickness control, the thickness of the extrudate is varied over the entire length in the direction of extrusion, the wall thickness being varied over the entire circumference of the extrudate. In the case of radial wall thickness control, a circumferentially based wall thickness profile can be produced over the entire length of the extrudate or over part of the length of the extrudate. This may be performed on the one hand by centering and decentering the mandrel with respect to the die body or the die body with respect to the mandrel, on the other hand this may be accomplished by additionally feeding material into the melt channel of the extrusion head.
In the case of a method described in DE 28 23 999 C2, it is envisaged to vary the gap width of the die gap during the discharge of the tubular parison by program-controlled axial adjusting movement of the mandrel and the die body.
In the case of the extrusion head according to DE 199 29 381 C2, an elastically deformable sleeve in the form of a thin-walled metal part is provided within the extrusion head or within the die and is deformed when setting the annular die by means of adjusting elements, so that a specific melt channel profile is produced in this way. Thus, the distribution of the material on the finished extrudate can be adapted relatively exactly to the requirements of the product to be produced, in particular the desired wall thickness distribution on the product.
All known methods in which it is not envisaged to feed in further melt streams in the region of the die gap (soling) have the disadvantage that varying the cross-sectional profile of the melt channel over part of the circumference is to the detriment or benefit of the width of the melt channel at another point of the circumference. With respect to the example of the deformable sleeve, this means that a constriction of the melt channel at one point of the circumference inevitably involves a widening of the melt channel at another point of the circumference, and vice versa.
As a result, there are in principle limits to the setting of the cross-sectional profile of the melt channel.
The invention is therefore based on the object of improving an annular die of the type referred to above and of providing a method that does not have the aforementioned disadvantages.